Labels bearing information in any of a number of different bar code formats are commonly affixed to products, packaging, or other items and are used in many applications. It is common, for example, to encode retail product identification information in a bar code format on a product package or a label affixed to a product package. Bar code symbols are also used on a broad range of retail packages for check-out and inventory purposes. A scanner, located at the check-out station of a retail establishment, may be used by a clerk to enter product identification data into an associated point-of-sale computer system.
Bar codes typically consist of a series of parallel light and dark rectangular areas of varying widths. The light areas are often referred to as "spaces" and the dark areas as "bars". Usually, a minimum element width called a "module" is e defined, with the bars and spaces arranged and selected to define different characters of a particular bar code.
A bar code label is typically read by a scanner which detects reflected and/or refracted light from the bars and spaces comprising the characters. Such a scanner includes a light source for illuminating the bar code label to be read. One common method of illuminating the bar code label is by use of a scanning laser beam, in which case a spot of light is swept across the bar code label and the intensity of returned light is detected by an optical detector. The optical detector generates an electrical signal having an amplitude determined by the intensity of the detected light. Another method for illuminating the bar code label is by use of a uniform light source with an array of optical detectors connected to an analog shift register (commonly called a charge-coupled device or CCD). In such a technique, as with a scanning laser, an electrical signal is generated having an amplitude determined by the intensity of the collected light. In either the laser or CCD technique, the amplitude of the electrical input signal has one level for dark bars and a second level for light spaces. As a label is scanned, positive-going transitions and negative-going transitions in the electrical signal occur, signifying transitions between bars and spaces. The electrical signal is converted to a binary scan signal which is analyzed to determine the arrangement of bars and spaces of the scanned label. The bar and space information is provided to a decoding unit to determine whether the bar code is recognized and, if so, to decode the information contained in the bar code.
While the use of bar codes greatly enhances the efficiency of the check-out process in the retail establishment and additionally allows the accumulation of sales or inventory data which is important for proper management control, difficulties may be encountered due to the number of different bar codes currently in use. Because bar codes in many different formats may need to be read by a scanner at a particular location, the scanner circuitry must be capable of recognizing and decoding labels printed in each of the codes used or expected in a particular application (an ability referred to herein as "autodiscrimination"). This requirement presents substantial difficulties since bar codes in common usage vary significantly in their formats. These codes include, for example, Code 3 of 9, Interleaved 2 of 5 Code, Codabar, Code 93, Code 128, the Universal Product Code (UPC), and the European Article Numbering (EAN) code. Each bar code has its own protocol and requires a separate, unique decoding algorithm. Thus, the amount of time required to perform the decoding algorithms increases as the decoder is required to discriminate among more label types. Further, the scanning speed of the moving laser beam is sufficiently high that the scanner must rapidly analyze the electrical binary scan signal to determine whether a bar code label has been scanned and, if it has, must rapidly analyze the data encoded on the bar code label. Also, it is important that the scanner system be capable of accomplishing these tasks automatically, without intervention by a clerk or other user, even when labels having different bar codes are affixed to products which are scanned.
Performance limitations in present scanners result in the possibility of relatively long decode times and lost decoding opportunities. The decoding unit of a scanner typically comprises, among other things, a processor (e.g., an 8-bit microprocessor) and an area of active memory for running decoding algorithms during the decoding process. The decoding unit has the ability to recognize a particular bar code format if the decoding algorithm for that format is present in the active memory of the decoding unit. Because of limitations in size and processing speed of decoding units, however, particularly in handheld scanners, not all decoding algorithms may be run in active memory at once. Usually, only a single decoding algorithm may be active in the decoding unit at a given time. Because the scanner does not know in advance which format a scanned label is in, the decoding unit typically rotates serially through the available decoding algorithms until the bar code format is recognized, if at all. If the label is not recognized, the read is determined to be unsuccessful. Less common bar code formats are usually given a lower priority for decoding purposes. If there are many available decoding algorithms, decoding speed may suffer if the correct bar code format is one of the later ones to be tried by the decoding unit. Consequently, the read rate varies depending on the priority of the correct bar code format. Because the available bar code formats remain in a fixed ranking, read rates will generally be slow for low priority, unusual, or long bar codes.
Speed in a bar code scanner is important because available processing time is short, especially in handheld scanners. Because the noise present in the optical and analog electronic signal paths can corrupt the data in a given scan, a number of scans of data are usually necessary before a good read decision can be made. Typical scan frames, or the time between data reads, in handheld scanners is about 25-35 milliseconds. The time between the end of one data set and the beginning of the next data set is perhaps 5-10 milliseconds. Because the same hardware is commonly used for both data collection and decoding, only this 5-10 millisecond period is typically available for decoding. If the decoder cannot finish all of the decode algorithms in this interval, one or more sets of subsequent scan data may be missed to allow decoding to finish. Long decoding times are particularly likely where there are many bar code formats being used or where one or more of the formats are of a long label type. For example, it may require as much as 40 milliseconds or more to decode a 40-character Code 128 label.
It may be possible to make use of faster hardware (e.g., higher-bit microprocessors) to attempt to speed up decode time. However, use of faster hardware may entail significantly greater cost, while the market for handheld scanners is very cost sensitive. Further, even faster hardware may not be able complete all decoding in a single scan frame. Faster hardware is also generally larger in size, requiring larger, bulkier handheld scanners and a corresponding sacrifice of ergonomics (e.g., size and shape of features). Since the power of digital processing hardware that can be built into a portable scanner has substantial size and cost limitations, software algorithms that can speed up the time to perform the decode operation are desirable to improve decoder performance.